In ionographic devices such as that described by U.S. Pat. No. 4,524,371 to Sheridon et al. or U.S. Pat. No. 4,463,363 to Gundlach et al., an ion producing device generates ions to be directed past a plurality of modulation electrodes to an imaging surface in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber in an imaging head, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on a charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices operate similarly, but do not rely on a moving fluid stream to carry ions to a surface, such as U.S. Pat. No. 4,763,141 to Gundlach et al. Some such devices provide low amounts of airflow for the purpose of removal of corona effluents.
Corona efficiency in ionographic heads is very low, on the order of 0.1% to 0.5%, when efficiency is defined as the ratio of the current reaching the electroreceptor to the total current within the corona chamber. The low efficiency limits the amount of charge that can be deposited on an imaging surface over a given period. Thus, in high speed devices, the development process is compromised because the development charge is relatively low, in comparison to other electrostatographic processes. One cause of this low efficiency is that the electric fields within the imaging head are not optimized to direct ions out of the ion chamber, so that ions instead contact conductive surfaces within the head where the ions neutralize into uncharged, or neutral air molecules. This can be partially overcome by increasing fluid flow velocity through the head to entrain ions within the fluid stream and carry the ions out of the head therein. One limitation on this method of improving corona efficiency is the increasing machine noise accompanying increased fluid flow. Dirt management and the high cost of the larger capacity air flow device are other problems. It would be more desirable to change the electric field shapes within the head, and particularly at the ion chamber exit, to direct a greater proportion of ions out of the head, without a requirement for a fluid jet, or enhancing the action of a fluid jet.
U.S. Pat. No. 4,593,994 to Tamura et al. discloses an ion flow modulator for use in a photocopying machine, including a common electrode formed on one major surface of an insulating substrate and a plurality of ion control electrodes formed on the other major surface of the insulating substrate.